This book gathers the proceedings of the 15th REHVA HVAC World Congress (CLIMA), held in Milan, Italy, on June 4-6, 2025, whose theme was "Decarbonized, healthy, and energy-conscious buildings in future climates". It covers a diverse range of topics such as new HVAC components and systems, HVAC impact on comfort and health of occupants and operators, breakthrough design approaches for minimization of HVAC carbon footprint, renewable energy communities plus energy districts, energy storages for the decarbonization of heating and cooling in buildings and districts, zero emission buildings, adaptive and integrated technologies for the building envelope, options for mitigating the impact of building energy consumption on future climate changes, and smart buildings. The contributions, which were selected by means of a rigorous international peer-review process, present a wealth of exciting ideas that will open novel research directions and foster multidisciplinary collaboration among different specialists.